This invention relates to an image readout device, and more particularly it relates to a two-dimensional image readout device in which sensor elements are disposed in a two-dimensional manner.
In the past years, MOS image sensors, CCD sensors, and contact type image sensors were proposed as devices to read out an image for use in telecopier or facsimile machines. The MOS type image sensor is a device by which incident images are scanned with an MOSIC and converted into electric signals through opto-electric transducers. The CCD sensor is a device in which light-induced charges at opto-electric converting areas are transferred through charge-coupled elements (CCD).
It is, however, inconvenient from functional and economical points of view to make an MOS type or CCD type sensor of a large area since those sensors rely upon the IC technique for buildup thereof on a mono-crystalline substrate. Generally, an optical lens is used for scaling-down and focusing of an image of the original onto the sensor. This method has the disadvantages that an optical path becomes longer and it is difficult to reduce the machine to a compact size because of the use of the optical lens.
To improve the above scaling-down/focusing type of image sensor, the contact type image sensor has been proposed which uses a sensor of the same size as the original and optical fiber for focusing an image of the same size as the original. For the contact type image sensor, an opto-electric converting area of the same dimensions as the original is necessary which requires deposit of a uniform layer over a large area. As an example of the contact type of image sensor, a one-dimensional contact type has been suggested which has a CdS photoconductive layer divided into individual islands. This device however, is complicated in structure because the photoconductive layer is divided during manufacture and individual electrodes need to be provided for the respective islands of the photoconductive layer. Another outstanding problem with this device is that readout speed is limited mainly due to light response speed of the photoconductive layer. Especially, the CdS photoconductive layer is poor in light response speed and applicability of this device as a one-dimensional image sensor is therefore substantially restricted.
Furthermore, another contact type of sensor has been proposed which has a strip-shaped photoconductor using amorphous silicon. If a two-dimensional image is to be read out with this type of one-dimensional sensor, then the sensor shall be repeatedly used and the same portion of the sensor shall be supplied with a voltage in a cyclic manner for readout of signals. For this reason, current variations responsive to brightness variations between "light" and "dark", that is, rising and falling characteristics directly affect readout signals and impose limits on readout speed. Those outstanding problems shall be solved by putting two-dimensional image sensors into practical use.